1. Field of the Invention
The present invention relates to a method and system for calibrating gas flow dilutors.
Standard gas mixtures are available in gas cylinders containing a fixed concentration of an analyte gas (for instance carbon monoxide) in a matrix gas (for instance nitrogen). Gas flow dilutors combine a flow of standard gas mixture with an additional flow of the relevant matrix gas (referred to as diluent gas) to provide a dilution. Adjustment of the ratio of the two flow rates provides a means of adjusting the concentration of analyte in the combined flow.
The present invention seeks to provide improved calibration of gas dilutors of a type which functions by turning “on” or “off”, in various combinations, a number of fixed flows of standard gas mixture and diluent gas. Such gas dilutor types are well known and often use capillaries, orifices or critical orifices as the fixed flow controlling components.
Gas dilutors are generally calibrated by measuring the actual flows of the flow controlling components with respect to external reference standards of flow. Such calibrations of commercially available gas dilutors are often performed annually, in calibration laboratories. This is expensive, time consuming and the accuracy may not be maintained throughout this period.
2. Description of the Related Art
However, calibration of actual flows is unnecessary since dilution is a function of flow ratios and knowledge of actual flow values is not required. British patent GB-B-2,333,614 describes a method of calibrating a number of flow controlling devices to accurately set their flow ratios by adjusting a series of flow combinations to give equal readings on a common flow meter. A detailed description of a “high-accuracy gas flow dilutor using mass flow controllers with binary weighted flows”, which utilises this method, was published in Measurement Science and Technology 13 (2002) 1138-1145. The method may be used to perform calibrations practically at frequent intervals at the point of use.
The method of British patent GB-B-2,333,614 is only applicable to dilutors which use flow-controlling components that are adjustable and can be set to a desired flow, such as mass flow controllers. Mass flow controllers are large, expensive and slow to stabilise to an adjustment of their flow. An accurate calibration, which depends upon adjusting flows to be equal, can therefore be protracted.